In the field of data communication, it is known to provide networks for transporting data units that have a logical structure according to which transport systems (such as e.g. Local Area Networks or LANs) are interconnected by bridge nodes, in order to form one overall network for transporting data units to and from any end nodes that are communicating with any one of the transport systems. The bridge nodes comprise two or more ports, each connected to a transport system. FIG. 2 shows a schematic example of such a network, in which 22, 23, 24, 26 and 28 denote bridge nodes, P denotes a port, and 21, 25, 27, 29 and 30 denote LANs as examples of transport systems. End nodes are not shown for simplicity. FIG. 3 shows a different configuration of bridge nodes 31 to 37, namely a ring configuration. The lines between the ports P represent the transport systems interconnected by the bridge nodes. Each of the shown examples can be a complete network or only a part of a larger system of bridge nodes and transport systems. For example, the bridge nodes 22, 24, 26 and 28 in FIG. 2 together form a ring configuration, and the nodes shown in FIG. 3 could be such ring in a larger system.
A controller in each bridge node handles the data units received at a given port, in order to determine the port out of which to forward the data unit, or whether e.g. to drop the data unit (for example because it is defective). The operation of forwarding a data unit can be done with the help of a record that associates data unit address information with port identification information. The controller then queries the record for identifying the port out of which to send a received data unit, using the address information in the data unit as a reference.
It is furthermore known to provide a learning procedure in the bridge nods, according to which the bridge node's record is updated on the basis of source address information contained in data units received at the ports. Namely, these source addresses are associated in the record with an identifier of the port at which the data unit arrived. If later a data unit arrives having that address as a destination address, then the data unit can be correctly forwarded to the correct port.
An example of a network using bridge nodes in this way is a layer 2 network using Medium Access Control (MAC) bridges according to IEEE standard 802.1 D. The filtering databases (FDBs) used in such MAC bridges are examples of the above mentioned records.
In networks of the above mentioned kind, which use records for handling data units in bridge nodes, it is furthermore known to purge such records in the event of a network topology reconfiguration. In other words, when the topology changes, e.g. due to a failure or shutting-down of a component (such as a bridge node or a transport system), then the information in the records becomes obsolete and should be reset.
In known systems, such as according to the IEEE 802 standard family, the reconfiguration is performed by first detecting the occurrence of a topology reconfiguration condition (e.g. that a transport system is disabled), then changing the topology and finally purging or flushing the records in the bridge nodes. Using the learning procedure, these records can then be built up again for the new topology.
During the process of reconfiguration, the handling of data units in the bridge nodes is continued, but with the old records derived on the basis of the old topology, such that data units are not correctly routed and effectively traffic is disrupted. This period of disruption of the network is also referred to the total recovery time of the network for reconfiguration. It comprises the time for detecting the topology reconfiguration condition, the time for letting the network converge to a new topology and the time for purging the records.
The object of the present invention is to provide an improved system and control method for the above described types of network, especially in view of making it possible to reduce the total recovery time.